Thermoplastic resins including a polyester are used in a variety of applications due to their excellent properties, e.g., mechanical properties, moldability, heat resistance, weather resistance, light resistance and chemical resistance. For example, a polyester film is widely used in a variety of applications, e.g., for electric and electronic materials including a magnetic recording medium, an electronic mount board and a capacitor, for packing, for medical use, and for various industrial materials, due to excellent properties of the polyester, e.g., heat resistance, mechanical properties, electric properties, chemical resistance, optical properties and environmental resistance. However, along with technological improvements made in recent years, better properties have been increasingly demanded according to applications in which the polyester is used. For example, to use the polyester for electric and electronic materials including a magnetic recording medium and an electronic mount board, improvements in the mechanical properties including elastic modulus, dimensional stability, surface properties and other properties are desired.
Heretofore, as a measure to improve these properties, a film processing technique as typified by orientation by stretching has been employed to make full use of properties inherent in the polyester (refer to page 2 in JP-A 2002-370276(the term “JP-A” as used herein means an “unexamined published Japanese patent applications”)).
However, with the conventional method, it is difficult to achieve properties better than those inherent in the polyester.
Meanwhile, recently, a so-called “nanocomposite” which is a composition prepared by dispersing a layered compound in a thermoplastic resin on a nano scale has been attracting attention. By forming the nanocomposite, properties improved over properties inherent in the resin have been attained with respect to various properties including high heat resistance, high elasticity, flame retardancy and a gas barrier property (refer to “World of the Nanocomposite”, Sumi Nakajyo, Kogyo Chousakai, 2000).
As for nanocomposite films as well, a polyamide film (refer to page 2 in JP-A 2000-336186), a polyimide film (refer to page 2 in JP-A 2000-7912), a polycarbonate film (refer to page 2 in JP-A 2001-131400) and the like are known, and improvements in properties such as a gas barrier property are disclosed. However, in the case of the polyester film, few reports have been made because dispersion of a layered compound is very difficult. For example, a layer inorganic substance containing film using a silane clay complex as a layered compound and a polyester as a resin is disclosed (refer to page 2 in JP-A 11-71509).
However, the film has many unclear points since, for example, there are no descriptions about side reactions which are often seen at the time of formation of the nanocomposite (for example, in the case of the polyester, production of a dialkylene glycol chain (refer to page 14 in JP-A 2002-514265)). A polyester composition using a silane clay complex is described in Table 4 on page 59 of WO 00/60006. The polyester composition has significantly lower mechanical properties including flexural modulus than an ordinary polyester.
As described above, the polyester nanocomposite film is still in development, and progress in its development has been desired.
Meanwhile, as the layered compound used in the polymer nanocomposite, a layered compound cation-exchanged with an organic ammonium ion is generally used. However, the thermal stability of the layered compound is not so high, and the layered compound therefore cannot be used for a resin having a high molding temperature (refer to page 2 in JP-A 11-1605). As a measure to solve the problem, use of an onium ion having a high thermal decomposition temperature such as a phosphonium ion (refer to JP-A 11-1605) or a hetero aromatic ion (refer to page 2 in JP-A 8-337414) as a cation exchanger is known. Although these cation exchangers can solve the problem of thermal decomposition, organic groups contained in these cation exchangers are a general long-chain alkyl group or aromatic group, and compatibility with the resin is not considered. As a result, the compatibility of the layered compound with the resin is not so high. As examples with improved compatibility with the resin, an example using a polyalkoxylated ammonium ion as a cation exchanger for a layered compound (refer to page 2 in JP-A 2002-514265), an example using a layered compound cation-exchanged with a long-chain alkyl ammonium ion in combination with a polyimide having high compatibility with a polyester (refer to page 2 in JP-A 2002-105294, for example) and an example incorporating a unit which can be a part of the polymer into a cation exchanger (refer to Y. Imai et al., Chem. Mater., Vol. 14, pp. 477 to 479, 2002) are known.
However, in the case of JP-A 2002-514265, there is a problem with respect to heat resistances of a polyalkoxyl group and an ammonium ion. Further, in the case of JP-A 2002-105294, the polyimide must be used in a rather excessive amount for the layered compound, thereby causing a possibility that properties inherent in the matrix resin may be impaired. In addition, in the case of Chem. Mater., Vol. 14, pp. 477 to 479, 2002, there is formed a structure that the polymer appears to be crosslinked on the surface of a layered compound where the cation exchangers gather, thereby causing a possibility of deterioration in tenacity. Further, due to the characteristic of the reaction, a short-time process such as melt blending cannot be used, and the range of application of the production process is narrow. As described above, formation of a nanocomposite from a thermoplastic resin and development of improved properties by use of a layered compound still have many problems, and improvements thereof have been desired.